Troubleshooting build failures
uv needs to build packages when there is not a compatible wheel (a pre-built distribution of the package) available. Building packages can fail for many reasons, some of which may be unrelated to uv itself.
Recognizing a build failure
An example build failure can be produced by trying to install and old version of numpy on a new, unsupported version of Python:
$ uv pip install -p 3.13 'numpy<1.20'
Resolved 1 package in 62ms
× Failed to build `numpy==1.19.5`
├─▶ The build backend returned an error
╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel()` failed (exit status: 1)
[stderr]
Traceback (most recent call last):
File "<string>", line 8, in <module>
from setuptools.build_meta import __legacy__ as backend
File "/home/konsti/.cache/uv/builds-v0/.tmpi4bgKb/lib/python3.13/site-packages/setuptools/__init__.py", line 9, in <module>
import distutils.core
ModuleNotFoundError: No module named 'distutils'
hint: `distutils` was removed from the standard library in Python 3.12. Consider adding a constraint (like `numpy >1.19.5`) to avoid building a version of `numpy` that depends
on `distutils`.
Notice that the error message is prefaced by "The build backend returned an error".
The build failure includes the [stderr] (and [stdout], if present) from the build backend that
was used for the build. The error logs are not from uv itself.
The message following the ╰─▶ is a hint provided by uv, to help resolve common build failures. A
hint will not be available for all build failures.
Confirming that a build failure is specific to uv
Build failures are usually related to your system and the build backend. It is rare that a build failure is specific to uv. You can confirm that the build failure is not related to uv by attempting to reproduce it with pip:
$ uv venv -p 3.13 --seed
$ source .venv/bin/activate
$ pip install --use-pep517 'numpy==1.19.5'
Collecting numpy==1.19.5
Using cached numpy-1.19.5.zip (7.3 MB)
Installing build dependencies ... done
Getting requirements to build wheel ... done
ERROR: Exception:
Traceback (most recent call last):
...
File "/Users/example/.cache/uv/archive-v0/3783IbOdglemN3ieOULx2/lib/python3.13/site-packages/pip/_vendor/pyproject_hooks/_impl.py", line 321, in _call_hook
raise BackendUnavailable(data.get('traceback', ''))
pip._vendor.pyproject_hooks._impl.BackendUnavailable: Traceback (most recent call last):
File "/Users/example/.cache/uv/archive-v0/3783IbOdglemN3ieOULx2/lib/python3.13/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 77, in _build_backend
obj = import_module(mod_path)
File "/Users/example/.local/share/uv/python/cpython-3.13.0-macos-aarch64-none/lib/python3.13/importlib/__init__.py", line 88, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
File "<frozen importlib._bootstrap>", line 1310, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
File "<frozen importlib._bootstrap_external>", line 1022, in exec_module
File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
File "/private/var/folders/6p/k5sd5z7j31b31pq4lhn0l8d80000gn/T/pip-build-env-vdpjme7d/overlay/lib/python3.13/site-packages/setuptools/__init__.py", line 9, in <module>
import distutils.core
ModuleNotFoundError: No module named 'distutils'
Important
The --use-pep517 flag should be included with the pip install invocation to ensure the same
build isolation behavior. uv always uses build isolation by default.
We also recommend including the --force-reinstall and --no-cache options when reproducing
failures.
Since this build failure occurs in pip too, it is not likely to be a bug with uv.
If a build failure is reproducible with another installer, you should investigate upstream (in this
example, numpy or setuptools), find a way to avoid building the package in the first place, or
make the necessary adjustments to your system for the build to succeed.
Why does uv build a package?
When generating the cross-platform lockfile, uv needs to determine the dependencies of all packages,
even those only installed on other platforms. uv tries to avoid package builds during resolution. It
uses any wheel if exist for that version, then tries to find static metadata in the source
distribution (mainly pyproject.toml with static project.version, project.dependencies and
project.optional-dependencies or METADATA v2.2+). Only if all of that fails, it builds the
package.
When installing, uv needs to have a wheel for the current platform for each package. If no matching wheel exists in the index, uv tries to build the source distribution.
You can check which wheels exist for a PyPI project under “Download Files”, e.g.
https://pypi.org/project/numpy/2.1.1.md#files. Wheels with ...-py3-none-any.whl filenames work
everywhere, others have the operating system and platform in the filename. In the linked numpy
example, you can see that there are pre-built distributions for Python 3.10 to 3.13 on MacOS, Linux
and Windows.
Common build failures
The following examples demonstrate common build failures and how to resolve them.
Command is not found
If the build error mentions a missing command, for example, gcc:
× Failed to build `pysha3==1.0.2`
├─▶ The build backend returned an error
╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel` failed (exit status: 1)
[stdout]
running bdist_wheel
running build
running build_py
creating build/lib.linux-x86_64-cpython-310
copying sha3.py -> build/lib.linux-x86_64-cpython-310
running build_ext
building '_pysha3' extension
creating build/temp.linux-x86_64-cpython-310/Modules/_sha3
gcc -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -fPIC -DPY_WITH_KECCAK=1 -I/root/.cache/uv/builds-v0/.tmp8V4iEk/include -I/usr/local/include/python3.10 -c
Modules/_sha3/sha3module.c -o build/temp.linux-x86_64-cpython-310/Modules/_sha3/sha3module.o
[stderr]
error: command 'gcc' failed: No such file or directory
Then, you'll need to install it with your system package manager, e.g., to resolve the error above:
Tip
When using the uv-managed Python versions, it's common to need clang installed instead of
gcc.
Many Linux distributions provide a package that includes all the common build dependencies. You can address most build requirements by installing it, e.g., for Debian or Ubuntu:
Header or library is missing
If the build error mentions a missing header or library, e.g., a .h file, then you'll need to
install it with your system package manager.
For example, installing pygraphviz requires Graphviz to be installed:
× Failed to build `pygraphviz==1.14`
├─▶ The build backend returned an error
╰─▶ Call to `setuptools.build_meta.build_wheel` failed (exit status: 1)
[stdout]
running bdist_wheel
running build
running build_py
...
gcc -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall -fPIC -DSWIG_PYTHON_STRICT_BYTE_CHAR -I/root/.cache/uv/builds-v0/.tmpgLYPe0/include -I/usr/local/include/python3.12 -c pygraphviz/graphviz_wrap.c -o
build/temp.linux-x86_64-cpython-312/pygraphviz/graphviz_wrap.o
[stderr]
...
pygraphviz/graphviz_wrap.c:9: warning: "SWIG_PYTHON_STRICT_BYTE_CHAR" redefined
9 | #define SWIG_PYTHON_STRICT_BYTE_CHAR
|
<command-line>: note: this is the location of the previous definition
pygraphviz/graphviz_wrap.c:3023:10: fatal error: graphviz/cgraph.h: No such file or directory
3023 | #include "graphviz/cgraph.h"
| ^~~~~~~~~~~~~~~~~~~
compilation terminated.
error: command '/usr/bin/gcc' failed with exit code 1
hint: This error likely indicates that you need to install a library that provides "graphviz/cgraph.h" for `[email protected]`
To resolve this error on Debian, you'd install the libgraphviz-dev package:
Note that installing the graphviz package is not sufficient, the development headers need to be
installed.
Tip
To resolve an error where Python.h is missing, install the python3-dev package.
Module is missing or cannot be imported
If the build error mentions a failing import, consider disabling build isolation.
For example, some packages assume that pip is available without declaring it as a build
dependency:
× Failed to build `chumpy==0.70`
├─▶ The build backend returned an error
╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel` failed (exit status: 1)
[stderr]
Traceback (most recent call last):
File "<string>", line 9, in <module>
ModuleNotFoundError: No module named 'pip'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<string>", line 14, in <module>
File "/root/.cache/uv/builds-v0/.tmpvvHaxI/lib/python3.12/site-packages/setuptools/build_meta.py", line 334, in get_requires_for_build_wheel
return self._get_build_requires(config_settings, requirements=[])
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/root/.cache/uv/builds-v0/.tmpvvHaxI/lib/python3.12/site-packages/setuptools/build_meta.py", line 304, in _get_build_requires
self.run_setup()
File "/root/.cache/uv/builds-v0/.tmpvvHaxI/lib/python3.12/site-packages/setuptools/build_meta.py", line 522, in run_setup
super().run_setup(setup_script=setup_script)
File "/root/.cache/uv/builds-v0/.tmpvvHaxI/lib/python3.12/site-packages/setuptools/build_meta.py", line 320, in run_setup
exec(code, locals())
File "<string>", line 11, in <module>
ModuleNotFoundError: No module named 'pip'
To resolve this error, pre-install the build dependencies then disable build isolation for the package:
Note you will need to install the missing package, e.g., pip, and all the other build
dependencies of the package, e.g, setuptools.
Old version of the package is built
If a package fails to build during resolution and the version that failed to build is older than the
version you want to use, try adding a constraint with a
lower bound (e.g. numpy>=1.17). Sometimes, due to algorithmic limitations, the uv resolver tries
to find a fitting version using unreasonably old packages, which can be prevented by using lower
bounds.
For example, when resolving the following dependencies on Python 3.10, uv attempts to build an old
version of apache-beam.
× Failed to build `apache-beam==2.0.0`
├─▶ The build backend returned an error
╰─▶ Call to `setuptools.build_meta:__legacy__.build_wheel` failed (exit status: 1)
[stderr]
...
Adding a lower bound constraint, e.g., apache-beam<=2.49.0,>2.30.0, resolves this build failure as
uv will avoid using an old version of apache-beam.
Constraints can also be defined for indirect dependencies using constraints.txt files or the
constraint-dependencies setting.
Package is only needed for an unused platform
If locking fails due to building a package from a platform you do not need to support, consider limiting resolution to your supported platforms.
Package does not support all Python versions
If you support a large range of Python versions, consider using markers to use older versions for
older Python versions and newer versions for newer Python version. For example, numpy only
supports four Python minor version at a time, so to support a wider range of Python versions, e.g.,
Python 3.8 to 3.13, the numpy requirement needs to be split:
Package is only usable on a specific platform
If locking fails due to building a package that is only usable on another platform, you can provide dependency metadata manually to skip the build. uv can not verify this information, so it is important to specify correct metadata when using this override.